Re: object algebra

"Eric Kaun" <ekaun_at_yahoo.com> wrote in message news:5BO%b.18056$2%4.355_at_newssvr31.news.prodigy.com...
> "Neo" <neo55592_at_hotmail.com> wrote in message
> news:4b45d3ad.0402261256.783f8bd2_at_posting.google.com...
> > However, if we allow EYE_COLOR = {Blue, Gaping_Unblinking_Sockets, NA,
> > UNK} we can no longer use isa, but instead a weaker relationship such
> > as isValueOf.
>
> Why? In Date's Tutorial D, you'd define different selectors. One of the
> problems (among many) with null is that it's untyped, whereas
> EYE_COLOR("N/A") and INTELLIGENCE_QUOTIENT("N/A") are of different types.
>
> > Thus Blue isValueOf EYE_COLOR.
> > Gaping_Unblinking_Sockets isValueOf EYE_COLOR.
> > NotApplicable isValueOf EYE_COLOR.
> > Unknown isValueOf EYE_COLOR.
> >
> > If Unknown can be a value of EYE_COLOR, why can't Rock be a value of
> > EYE_COLOR?
>
> Because you haven't defined it as such.
>
> > In the above example, the weaker relationship kills inheritance. You
> > could try to patch it with custom code, but it could not anticipate
> > the nature of all new values added in the future (ie pink, not_sure,
> > etc).
>
> Tutorial D's discussion of inheritance is extensive, and there's no such
> problem. Subtypes cannot violate their supertypes' invariants, unless I've
> missed something (note that this is unlike many O-O languages).
> Specialization by constraint does wonders here.
>
> > So are you in favor of NULLs (or its patches) and the resulting weaker
> > realtionships?
>
> The above isn't a "patch for" nulls. What does "weaker relationship" mean,
> though? I have no idea how to address that one.

The last bit would be the appropriate time to cite Date's Principle of Incoherence. Received on Fri Feb 27 2004 - 22:44:25 CET